Proteins which bind 6-substituted purines of the type which have cytokinin activity have been isolated from pumpkin cotyledons, beam embryos and wheat germ and extensively purified from the latter. Antibodies against these proteins are being used as a probe for the biological activity of these proteins. In addition, studies of the subunit structure of the protein are under way. Photoaffinity labeled cytokinins have been prepared and will be used in a study of the conformation of the active site.